The present research seeks to determine the significance of observed differences in nucleoside Q-containing tRNAs that have been associated with certain developmental and neoplastic states. I have purified a substance (designated Q-factor) found in mammalian sera which enables tissue culture cells to synthesize Q-containing tRNAs. Q-factor appears to be Q base. By controlling the level of Q-factor in media, cells can be grown which exhibit tRNA either modified or unmodified with respect to Q. The generality of this phenomenon will be examined, as well as the significance of Q-factor as a precursor of Q in tRNA under physiological conditions. The protein expression of cells grown in the presence and absence of Q-factor will be examined by two-dimensional polyacrylamide gel electrophoresis. The function of any Q-specific proteins will be sought, as well as the type of control, whether transcriptional or translational, involved in their synthesis. The synthesis of Q in mammalian cells will be studied by identifying and exploiting a suitable tissue culture or organ culture system. Utilizing radioactive guanine as a precursor, Q base and its intermediates will be monitored after separation by high pressure liquid chromatography.